Raindrops are formed when the cloud droplets grow big enough to fall out of the clouds. Most of the rain that falls in the winter, and even a lot of it that falls in the summer, is from melting of snowflakes as they fall through warmer air.

Clouds are formed when moist air rises upward. As the air rises, it becomes colder. Eventually the air can't hold all of the water vapor in it, and some of the water vapor condenses to form tiny water droplets. When moist air is cooled at the ground, fog is formed in the same way.

The wind blows because air pressure is higher in one place than in another place. The air moves from areas of higher pressure to areas of lower pressure. Objects like buildings, trees, and hills affect the direction of the wind near the surface. To get the best idea of the wind direction, try to stand far away from such objects. A park or a playing field is the best place to observe the wind.

Air consists of gas molecules, which are combinations of two or more atoms. Although you cannot see them with your eyes, the molecules are constantly moving this way and that at very high speeds. As they move, they collide with one another and with solid surfaces. The temperature of the air is a measure of how quickly the molecules are moving. The more energy of motion the molecules have, the higher the temperature you feel in the air.

All sciences begin with observations. Without observations, scientists have no way to develop new theories and to test existing theories. The weather is no exception. Meteorologists (scientists who study the weather) observe many elements of the weather, both at the Earth's surface and at high altitudes. Weather observations are used for predicting the weather and for developing and testing new theories about how the weather works.

Most of the water that is used in homes and businesses is put into either municipal sewers or home septic systems. Most of that water is polluted to some extent, because it comes from clothes washing, bathing, and toilets. In earlier times, sewage was put directly into the ground, into rivers, or into the ocean, without any treatment. As population has grown, however, the need for wastewater treatment has increased as well.

Some sources of drinking water are of such high quality that not much treatment is needed. Usually adding small amounts of chlorine is sufficient to kill any harmful bacteria or other microorganisms. Other water sources, especially large rivers, have higher levels of pollution. Such sources require more to bring the water up to the needed level of quality. River water usually contains fine sediment particles in suspension. The water can be passed through filtration materials, like sand, to remove the fine sediment. Filtering the water also tends to remove bacteria.

Most people in the United States get their water from municipal (city and town) water systems. Most people in rural areas, and some also in suburbs, get their water from their own wells, which tap shallow or deep groundwater. The water that is supplied from municipal water systems comes mainly from three sources: streams and rivers; natural lakes or artificial reservoirs; and ground water, pumped from large wells. Lakes and reservoirs that are located in unpopulated areas far from cities and towns usually have the highest-quality water.

Naturally occurring substances also affect water quality. Even raindrops are not pure water. As they fall, they pick up tiny dust particles and also harmful substances like acid that are in the atmosphere. When the rainwater comes in contact with soil and rock material, some of that material dissolves in the water. Substances that are dissolved in water are called solutes. The concentration of natural solutes depends mainly on two factors: the composition of the soil and rock material, and how long the water is in contact with that material.